The present invention relates generally to textile cords for pneumatic tires and more specifically to cords for belts of radial carcass tires for heavy duty use.
Radial carcass tires, especially off-the-road tires and heavy duty off-the-road tires have typically 1 to 5 carcass plies, 2 to 6 belt plies and one to four sets of beads.
Off-the-road tires, because of the nature of their use, are exposed to cutting and delamination caused by sharp rocks and uneven terrain. This problem is addressed by providing such tires with belts reinforced with high strength material.
One reinforcement cord currently used in belts of heavy duty industrial tires is composed of 3 yarns of aramid, having a linear density of 1500 Denier, the yarns and the cord being twisted at 6.9 turns per inch (TPI). Such cord has a strength of about 750 N. Aramid is high cost to manufacture and to process.
Another reinforcement cord currently used in belts of agricultural tires is composed of 2-3 yarns of rayon, having a linear yarn density of 1650 to 2200 Denier, the yarns and the cord being respectively twisted at about 9 TPI and 6.5 TPI. Such cords have strengths of about 180 N and 350 N respectively. This cord is high cost to manufacture because of high material cost, and the cord is subject to moisture pick-up and the cord has a low tenacity.
Polyester cords, such as 2.000/2 6/6 or 1.000/2 12/12 TPI which have respectively twist multipliers of 5.2 and 7.35 are also currently used to reinforce belt plies of farm tires. The high twist levels of between 6 and 12 TPI, however, do not take full advantage of the dimensional stability, the modulus, the shrinkage, and the tensile strength of the polyester material. The material cost for this material is also high.
It an object of the present invention to provide a cord with high tensile strength, high modulus, high dimensional stability and low gauge, while maintaining sufficient fatigue resistance, and which is less expensive to manufacture and to process than a cord used in the prior art.
A polyester cord for a belt of a pneumatic tire of the invention has a linear density of 2000 to 16000 Denier and comprises 2 to 9 yarns. The yarns have substantially the same twist as the cords and the cord has a twist of 2 to 7 TPI and a twist multiplier of 3 to 5. In one illustrated embodiment, the cord comprises 2 to 3 yarns, has a linear density of 2000 to 4000 Denier, a twist multiplier of 3.5 to 4.5 and a twist of 4.5 to 6.5 TPI. In another illustrated embodiment the cord comprises 3 to 9 yarns, has a Denier of 10000 to 15000, a twist multiplier of 3.5 to 4.5 and a twist of 2.2 to 3.2 TPI. In a third embodiment, the cord comprises 6-9 yarns, has a Denier of 11000 to 13000, a twist multiplier of 3,5 to 4,5 and a twist of 2.5 to 3.0 TPI.
The cord of the invention is preferably made of polyester such as poly-ethyleneterephthalate (PET) or poly-ethylenenaphthalate (PEN).
Also provided in the invention is a pneumatic tire comprising at least one radial carcass ply, a tread disposed radially outwardly of the crown region of the carcass and a crown reinforcing structure interposed between the tread portion and the crown region of the carcass in circumferential surrounding relation to the carcass. The crown reinforcing structure of the tire includes a belt assembly having at least a first, radially innermost and a second, radially outermost belt ply, each of the belt plies comprising reinforcement cords extending parallel to one another in each belt ply, and at least one belt ply is reinforced with cords made according to the invention. In an illustrated embodiment, the crown reinforcing structure of the tire comprises 2 belt plies wherein the cords of the belt plies have a linear density of 2000 to 4000 Denier and a lateral cord density of 20 to 30 EPI. In a second embodiment, the crown reinforcing structure comprises four belt plies wherein the cords have a linear density of 11000 to 13000 Denier and a lateral cord density in the belt plies of 10 to 15 EPI. In a third embodiment, the crown reinforcing structure comprises 2 belt plies and the cords have a linear density of 11,000 to 13,000 Denier and a lateral cord density in the belt plies of 10 to 15 EPI.
The fabric using the cords has a distance between two neighboring cords (rivet area) far above the minimum distance of 0.1 mm required for guaranteeing sufficient rubber flow between the cords during calendering; such rubber penetration between the cords is required for cord separation resistance. At the same time the fabric ensures sufficient tire reinforcement strength, which is defined by individual cord strength and lateral cord density.